1. Field of the Invention
The present invention relates to a straddle-type vehicle, and more specifically, it relates to an engine air-intake system for a straddle-type vehicle.
2. Description of the Related Art
In the engine air-intake system mounted in conventional straddle-type vehicles, a carburetor is disposed behind a cylinder head constituting a cylinder assembly of an engine arranged substantially upright, an air cleaner is disposed behind the carburetor, and an exhaust pipe constituting an engine exhaust system is disposed beside the engine air-intake system (see, for example, Japanese Unexamined Patent Laid-open Publication Nos. HEI 11-11171 and 2002-274468).
Since the exhaust pipe is located, as a heat source, in the vicinity of the carburetor and the air cleaner, temperature of outside air to be led into the engine increases. In addition, since the carburetor and the air cleaner are located behind the cylinder head, it is difficult to cool them by the wind while the vehicle is moving. The increase in the temperature of the air will reduce the air-intake efficiency, resulting in reduction of the engine power. Moreover, since the air-intake path from the air cleaner to the air-intake port in the cylinder head has a long length and is bent steeply just before the air-intake port, the air-intake resistance will be increased. The increase in the air-intake resistance may deteriorate the throttle response and the air-intake efficiency.
Furthermore, in the arrangement of the substantially upright cylinder assembly, the center of mass of the vehicle will take a high position, which will deteriorate the steering performance of the vehicle.